Environmental Effects of Historical Mining in the Animas River Watershed, Southwestern Colorado ecological recovery following remedia- tion implemented in the past decade by The U.S. Geological Survey has completed an extensive environmental study of the Sunnyside Gold, Inc., the Federal and effects of historical mining on water and sediment quality and aquatic and riparian State land-management agencies, and habitat in the Animas River watershed upstream from Silverton, Colorado. Results the Animas River Stakeholders Group. from this study are being used by Federal and State agencies and by the local A compact disc of data collected during watershed stakeholders group to implement remediation and cleanup activities. the study accompanies the volume. The Watershed Approach provides an effective means to rank and evaluate environmental impact of historical mining throughout a region: The mining-related Setting and Geology sites that, upon evaluation, are shown to cause the greatest impact can become the subjects of remediation planning and implementation. This process helps land The Animas River watershed study managers evaluate risk and focus remediation efforts on sites that may be the most area (fig. 1), approximately 145 square deleterious to water quality and aquatic and riparian habitat. miles, covers the northern portion of the Animas River drainage basin upstream The Animas River watershed is one and riparian biota. An environmental of Silverton, Colo.; it also includes an of many watersheds in the western United risk assessment for trout summarizes the extension south to Elk Park. Upstream States where historical mining has left a biological impact of historical mining on from Silverton, the drainage basins legacy of acid mine drainage that results aquatic fishery resources. Viable trout of the three main tributaries, Mineral in elevated concentrations of potentially populations are present in some tributaries Creek, Cement Creek, and Animas River, toxic trace elements. Many inactive mine and parts of the Animas River upstream delineate a rugged montane basin, rang- sites are located on or directly affect from the confluence with Mineral Creek, ing in elevation from 9,305 ft at Silverton Federal lands. Cleaning up these Federal suggesting some overall improvement of to more than 13,800 ft in the surrounding lands and restoring the watersheds aquatic habitat quality. However, little mountains. Elevations of the three Red will require a substantial investment of to no aquatic life occurs in the upper Mountains (banner photo at top) range resources and many years of work before Animas River upstream of Animas Forks, from 12,219 to 12,747 ft; these three the aquatic ecosystem can recover. As the entire length of Cement and Mineral peaks form the northwest boundary of part of a cooperative effort with Federal Creeks, and several smaller tributaries. the study area. The study area is accessi- land-management agencies, State natural Historically, parts of Cement and Mineral ble by U.S. Highway 550, from Durango resource agencies, the U.S. Environmental Creeks, the two major tributaries in the on the south or from Ouray on the north. Protection Agency, and concerned local study area, were always acidic, contained The study area is underlain by citizens, the U.S. Geological Survey high metal loads, and likely did not sup- Precambrian and Paleozoic rocks that (USGS) implemented the Abandoned port any aquatic life other than species of were intruded and overlaid by the Mine Lands Initiative in 1997. USGS algae that can tolerate those acidic condi- Oligocene Silverton Volcanics in a major scientists have since completed a compre- tions (pH<4). Finally, the Professional caldera-forming event. Ring fractures hensive study to understand processes and Paper outlines the potential for long-term control the course of the Animas River assess sites in the watershed for reme- monitoring as a means to document and Mineral Creek upstream from diation. These data, coupled with those Silverton. Mineralization in the watershed collected from private sites by the Animas was extensive, comprising at least five River Stakeholders Group, State of Animas River different periods beginning in the Oligo- Colorado, Bureau of Land Management, watershed study area Silverton cene, 30 million years ago, and ending and USDA Forest Service, will be used by about 5 million years ago. Hydrothermal regulatory agencies having the responsi- COLORADO SAN JUAN alteration has resulted in widespread bility for watershed cleanup. Animas River areas of disseminated pyrite throughout Results of this investigation, pub- watershed the study area, weathering of which has lished in USGS Professional Paper 1651, in Colorado LA PLATA resulted in large barren, iron-stained include studies of geology, rock and sedi- areas (banner photo) that release large ment geochemistry, surface- and ground- N Durango amounts of acid and metals to surface and water geochemistry, a mine inventory, ground water. district production and milling history, Ignacio and distribution and health of aquatic Figure 1. General location map of study 0 5 10 15 20 25 MILES area, southwestern Colorado, U.S.A. U.S. Department of the Interior Printed on recycled paper Fact Sheet 2007–3051 U.S. Geological Survey August 2007 Figure 2. Animas River valley at Eureka. Foundation of Sunnyside Eureka Mill is on hill at far left above road. Riparian zone downstream from the mill site was completely obliterated by discharge and storage of mill tailings during the mill’s operation (1917–1930). About 1 meter of sediment accumulated on the flood plain as a result of disposal of mill tailings in the Animas River. The site was remediated by Sunnyside Gold, Inc., in 1996. Photograph by Kirk Vincent (1999). The Mining Era stopes or on mine-waste dumps outside Environmental Effects the portal where the hand sorting was Sometime during the 1700s, done. As more efficient methods of min- Historical mining occurred during Spanish explorers discovered placer ing and milling were developed and rail a time when environmental effects of gold in Arrastra Creek, a tributary of the transport became available, increasingly mining were poorly understood and of upper Animas River. Serious mineral larger amounts of lower grade ore were little understanding or concern to much exploration began in the early 1870s fol- mined and processed. Waste rock was of society. Effects of mining are notice- able throughout the watershed (fig. 3). lowing discovery of placer gold by the disposed of on mine-waste dumps outside In many stream reaches, elevated trace- Baker reconnaissance scouting team for the mine portal, and mill wastes were element concentrations are the direct the U.S. Army. Mining began in 1872 deposited into the nearest stream course. By 1930, before it closed due to falling result of historical mining exemplified following a treaty with the Ute Tribe by zinc (fig. 4). Other potentially toxic and continued through 1991—when metal prices during the Depression, the Sunnyside Eureka Mill (fig. 2) was trace elements (Al, Cd, Cu, Pb, and Mn) the last remaining site, the Sunnyside in both water and sediment resulting processing more than 1,000 short tons of mine, closed. Mineral deposits include from historical mining activity show the ore per day. Mill wastes were disposed of copper- and silver-rich breccia-pipe same general distribution patterns. The on the riparian zone of the Animas River deposits associated with subeconomic presence of Paleozoic carbonate rocks as porphyry copper-molybdenum depos- and were periodically washed down- well as the carbonate in propylitic hydro- its, and base- and precious-metal vein stream by high spring runoff. Begin- thermal alteration in the upper Animas deposits containing silver, lead, zinc, and ning about 1935, mill wastes from the River basin resulted in good near-neutral gold in hydrothermal vein deposits. The Shenandoah-Dives mine were retained water in the Animas River and many most productive mineral deposits were in tailings ponds at the Mayflower Mill of its west-flowing tributaries (fig. 5). vein deposits mined primarily for their site. An estimated 8.6 million short tons Tracer studies conducted in the watershed silver content. of mill waste, about 47.5 percent of the indicate that the Animas River upstream Silver, lead, and gold were the total ore produced, were discharged from Cement Creek contributes the larg- primary commodities. Zinc was produced into surface streams between 1872 and est portion of the manganese. In contrast, in minor quantities prior to World War I, about 1935. water from Mineral Creek contributes but it became a major commodity follow- ing the introduction of pre-flotation pro- cessing in 1915. Zinc production expanded after the Sunnyside Eureka flotation mill opened in 1917, until the Great Depression forced the mine and mill to close. Much of the total mineral wealth extracted from the gold-bearing vein deposits came from the Shenandoah-Dives and Sunnyside mines in the upper Animas River basin. Total mineral production from the area is estimated at 18.1 million short tons of ore. Mining and milling processes in the Animas River watershed are typi- cal of those practiced throughout the West during the 19th and 20th centuries. Beginning in 1872, vein deposits were Figure 3. View looking downstream (southwest) from confluence of Cement Creek (right) mined underground by small crews who with Animas River (left). Iron colloids precipitate from more acidic surface water